Many mobile stations support dual or multi mode operation, i.e. support operation both in a first and in a second mode. Normally a mobile station operating in A or Gb mode will be connected to the core network via the A interface or the Gb interface respectively. The A interface is an interface between a BSS (Base Station Subsystem) and a 2G MSC (Mobile Switching Centre) whereas the Gb interface is an interface between a BSS and a 2G SGSN (Serving GPRS Support Node). In a similar manner, a mobile station operating in Iu mode will be connected to the core network via the Iu interface (Iu-ps is the interface between a BSS and a 3G MSC whereas Iu-cs is the interface between a BSS and a 3G SGSN).
For the GERAN (GSM/EDGE (Enhanced Data rates for Global Evolutions) radio access network) architecture a concept has been proposed with a 2G/3G capable cell, the associated RAN (Radio Access Network) of which may be connected to the core network using either the A/Gb interface or the Iu interface. In such a cell an MS is either operating in A/Gb mode, and is therefore connected to the core network via the A/Gb interface, or operating in Iu mode and is connected to the core network via the Iu interface.
Also for such a concept existing or conventional procedures for initial cell selection and cell reselection apply. Upon successful completion of cell selection/reselection, the mobile station must choose a mode of operation, assuming that the chosen cell is a combined 2G/3G capable cell.
Connection of a radio access network with the A/Gb interface and with the Iu interface to the core network are for example described in 3GPP TS 43.051 v6.0.0 (2003-11).
A GERAN cell can support A/Gb mode only, or Iu mode only, or both modes as discussed above. The support of each mode depends on the interfaces with which the GERAN is connected to core network nodes. The support of each mode of operation by a GERAN cell is indicated in broadcast system information and messages.
However, as far as mode selection is concerned, currently the Iu mode shall be selected in the target cell if Iu mode is supported by the cell and by the mobile station, unless otherwise ordered by the network.
If initial mode selection is fixed to one of the modes, if for example all Iu capable mobile stations are to select Iu mode in mixed mode cells as discussed above, the network may experience load problems which may strongly impact the performance. If for example a network is highly loaded and all Iu and A/Gb capable mobile stations have to select the Iu mode, Iu capable only mobile stations may be denied access to the network unless the network forces 2G-3G capable mobile stations to perform an inter-RAT handover. The signalling will also have a negative impact on network performance and possibly lead to service interruption. Such a solution is described in 3GPP TS 43.051 in section 5.3.
Alternatively or additionally unnecessary signalling will be introduced due to the implementation of a mode selection load sharing algorithm in the network which triggers inter-RAT (Radio Access Technology) handovers when required. Such signalling will have a negative impact on network performance and may lead to service interruption.